1. Field of the Invention
The present invention relates to torque wrenches of the type which include electronic means for measuring torque applied to a structure in response to application of force to the wrench by the operator.
2. Description of the Prior Art
Torque wrenches of the electronic type typically include an elongated deflection beam provided at one end with a head portion for coupling to an associated structure, such as a threaded fastener or the like, and provided at the other end with an elongated handle. Strain gauges are disposed on the deflection beam to measure the strain or deflection of the beam in response to forces applied to the handle by the operator in torquing the threaded fastener. Typically, the strain gauges are connected in an electronic circuit which produces an indication of the torque applied to the fastener as a function of the measurements made by the strain gauges.
The deflection beam is typically an elongated rectangular or cylindrical bar and a common arrangement utilizes four strain gauges connected in a Wheatstone bridge network. It is known that if these four gauges are arranged on the beam so that three of them are disposed at a first distance "a" from the coupling head, with two gauges on one side of the beam and another gauge on the opposite side of the beam, and the fourth gauge is disposed on the one side of the beam at a distance 3a" from the coupling head, the torque measurement will be independent of the position along the handle at which the force is applied by the operator, as long as that position is not closer to the coupling head than the fourth strain gauge. This is obviously a particularly advantageous arrangement, since it permits an operator to operate the wrench without being concerned as to the placement of the operator's hand on the handle.
In torque wrenches having a flex ratchet type coupling head, the strain gauges must be disposed away from the coupling head. Thus, the minimum value of the distance "a" in this arrangement is limited by the physical design of the wrench and the coupling head, and typically necessitates that the fourth strain gauge be disposed at a relatively large distance from the coupling means. Normally, additional length is needed to accommodate the associated electronic circuitry and, therefore, the foregoing arrangement may result in an inconveniently long torque wrench.
U.S. Pat. No. 4,006,629 to Barrett et al. discloses a torque wrench having four strain gauges arranged in a bridge network and all disposed on a reduced-thickness portion of the deflection beam of the wrench. The patent discloses that this arrangement can be utilized to produce torque measurements independent of the user's hand-hold position, but it necessitates that the strain gauges have different sensitivities or gauge factors.